Improve series functionality

[Tekki]

Pull Request type

Please check the type of change your PR introduces:

• Bug-fix
• Feature (functionality, design, translations, etc.)
• Documentation change
• Project management (tests, CI, GitHub configuration, etc.)
• Other (please describe):

Current state

Closes #430

Proposed changes

The PR consists of 3 commits:

1. Correction of the series entries in the example site; series is a single value, not an array.

2. Changes at the code. The entries in params.toml by the original author of the series functionality

# showRelatedInArticle = false
# showRelatedInSidebar = false

suggest that the default value of this variables is false. The code should behave accordingly.
Then I thought it could be useful if both variables can be overridden in a post.

3. Posts in series sorted by date ascending. In my opinion the articles in series build a sequence and are intended to be read in chronological order, oldest first.

Screenshots, if applicable

Checklist

• Bug-fixes and new features: I have tested locally with the latest release of Hugo extended. This requirement is a standard.
• Bug-fixes, new features, and doc changes: I have updated the relevant documentation as part of this PR.
• All PRs: I have signed off (using git commit -s ...), or if not possible due to developer environment constraints, will comment below confirming that I am adhering to the Developer Certificate of Origin.

[Tekki]

I am adhering to the Developer Certificate of Origin.

[chipzoller]

I know this is so late, @Tekki, but could you resolve conflicts here so we can try and get it in?

[chipzoller]

Thank you. Requesting a review from @onweru here.